1. Field of the Invention
The present invention relates to a magnet type rodless cylinder of a type comprised of a cylinder tube at the inside of which is formed a cylinder bore in which is arranged a piston so as to be able to move in the tube axial direction and at the outside circumference of which is arranged a single slide so as to be able to move in the tube axial direction, the piston and slide being magnetically connected, more particularly relates to a magnet type rodless cylinder where the cylinder tube has a noncircular shape, in particular has a flattened shape.
2. Description of the Related Art
As this type of magnet type rodless cylinder, for example, there is the one described in Japanese Utility Model Publication (A) No. 4-11305. The magnet type rodless cylinder of Japanese Utility Model Publication (A) No. 4-11305 reduces the thickness (height) of the cylinder or increases the pressure receiving area of the piston or increases the magnetic coercive force by making the cylinder tube and piston to flattened cross-sectional shapes in their diametrical directions. Further, Japanese Patent Publication (A) No. 4-357310 describes making the cylinder tube and piston elliptical or peanut-shaped cross-sectional shapes. Further, Japanese Utility Model Registration No. 2514499 discloses arranging two magnet type rodless cylinders in parallel and guiding a single slide spanning these two cylinders.
In the generally used magnet type cylinders, the cylinder tube and cylinder bore have true circular cross-sectional shapes. For this reason, when the tube is subjected to inside pressure, the tube will uniformly deform (expand) in cross-section, so the stress acting on the tube will also be uniform and no local concentrations of strain or stress will occur. As opposed to this, in a tube with a flat (noncircular) outside shape like in Japanese Utility Model Publication (A) No. 4-11305 and Japanese Patent Publication (A) No. 4-357310, the cylinder bore also has a noncircular cross-sectional shape, so if the tube is subjected to inside pressure due to fluid inside it, the tube will not deform uniformly. For this reason, when using a noncircularly shaped cylinder tube, the tube will be subjected to stress concentrations or local deformation and sometimes will have extremely large maximum stress and deformation.
To solve this problem, it may be considered to increase the tube thickness so as to raise the tube rigidity, but if increasing the tube thickness, it is necessary to commensurately increase the magnetic coercive force coupling the piston and slide. In this case, the required magnetic coercive force will sometimes be several times larger than the magnetic coercive force when using a tube with a circular cross-sectional shape. For this reason, while a magnet type rodless cylinder having a tube of a noncircular shape has existed as an idea, none has even been practically realized up to now.
On the other hand, while Japanese Utility Model Registration No. 2514499 describes two magnet type rodless cylinders arranged in parallel and a single slide provided for these two cylinders, this single slide is provided inside it with separate outside magnets or magnetic bodies corresponding to the respective cylinders. For this reason, the magnet type rodless cylinder of Japanese Utility Model Registration No. 2514499 has the problems of an increase in the number of parts and complicated assembly.
Further, in general conventional magnet type rodless cylinders, when the piston (that is, the inside magnets) moves due to inside pressure, the movement of the inside magnets causes the slide to be attracted and moved. The slide is moved by this mechanism. The size of the attraction force at this time is used as an indicator of the transport capacity of the magnet type rodless cylinder and is usually called the “magnetic coercive force”.
FIG. 6 shows in a simplified manner the cross-section of a general conventional magnet type rodless cylinder along the cylinder axis. Reference numeral 100 indicates a cylinder tube, while 101 indicates a slide arranged outside of the tube. As shown in FIG. 6, the slide 101 outside of the tube 100 is provided with four outside magnets 102, while the piston 103 inside the tube 100 is provided with four inside magnets 104—both in the axial direction. Further, the four magnets forming the outside magnets 102 and the four magnets forming the inside magnets 104 are arranged so that the same poles of the magnets face each other across the yokes 105 in the axial direction. The magnets of the inside magnets 104 and the magnets of the outside magnets 102 are arranged so that different poles face each other in the radial direction.
Here, the magnetic coercive force is defined as the axial direction force acting at the slide 101 in the state where the slide 101 is fixed so that it cannot move in the axial direction and when fluid pressure is applied to the piston 103 to make the inside magnets 104 displace in the axial direction with respect to the slide 101 (outside magnets 102). As shown in FIG. 5, in the stationary state where no fluid pressure is acting, that is, the state where of the four outside magnets, the outside magnets 104, 102 face each other in the radial direction and do not displace in the cylinder axial direction, as shown by the point A, the magnetic coercive force becomes zero. Further, the magnetic coercive force, as shown in point B of FIG. 5, becomes the maximum value Max when the relative displacement of the magnets 102, 104 becomes about half of the pitch of arrangement L of the magnets 102, 104 in the axial direction.
In this way, in a general magnet type rodless cylinder, stationary state, the inside magnets 104 and the outside magnets 102 attract each other in the radial direction and are aligned in the axial direction, so in the stationary state, the magnetic coercive force becomes zero. Therefore, if making this piston 103 move from this state, no magnetic coercive force acts until relative displacement occurs between the inside magnets and outside magnets in the axial direction. Even if the piston 103 moves, sufficient attraction force does not act on the outside magnets 102. For this reason, in a conventional magnet type rodless cylinder, at the time of start of operation from the stationary state, even if the piston 103 starts to move, the slide 101 will not start to move smoothly following this, that is, the so-called “stick-slip phenomenon” is seen, and other problems occur.
This problem naturally occurs in each magnet type rodless cylinder in two magnet type rodless cylinders arranged in parallel at a relatively large distance as in Japanese Utility Model Registration No. 2514499 and in cylinders provided with noncircularly shaped tubes as in Japanese Utility Model Publication (A) No. 4-11305 and Japanese Patent Publication (A) No. 4-357310.